Preamplifier

ABSTRACT

A preamplifier includes a negative-feedback amplifier circuit that converts a current signal from a photodetector into a voltage signal; and a conversion-gain control circuit that simultaneously controls a resistance value of a feedback resistor portion of the negative-feedback amplifier circuit and a resistance value of a load resistor portion of the negative-feedback amplifier circuit, based on the voltage signal from the negative-feedback amplifier circuit. Each of the feedback resistor portion and the load resistor portion includes a fixed resistor element, a MOSFET element, and a diode-connected transistor, connected in parallel.

TECHNICAL FIELD

The present invention relates to a preamplifier used in an opticalcommunication system and an optical receiver. More particularly, thepresent invention relates to a preamplifier with improved stability ofoperation and suppression of circuit saturation, or improved followingcharacteristics of gain control.

BACKGROUND ART

A preamplifier is provided in a front-end of an optical receiver and hasa function to convert a current signal to a voltage signal, the currentsignal being converted in a light receiving element that is a componentof the optical receiver. The optical receiver is used in a system havingvarious conditions such that the intensities of light received aredifferent. Therefore, the preamplifier provided in the front end of theoptical receiver requires wide dynamic range characteristics.

For this type of preamplifier, there has been an example of disclosureas a preamplifier having the wide dynamic range characteristics (see,for example, H. Ikeda, et al. “An Auto-Gain Control TransimpedanceAmplifier with Low Noise and Wide Input Dynamic Range for 10-Gb/sOptical Communication Systems”, IEEE J. Solid-State Circuits, vol. 36,pp. 1303-1308, 2001).

Disclosed in the above literature are a circuit configuration andvarious characteristics of the preamplifier having the wide dynamicrange characteristics.

In the preamplifier disclosed in the above literature, a feedbackresistor of a negative-feedback amplifier includes a parallel circuithaving mainly a fixed resistor and a field effect transistor (FET). Agate terminal voltage of the FET is changed according to light receivingpower to control a conversion gain, and a wide dynamic range isrealized. If the light receiving power is low, the FET is completelyOFF, and the preamplifier operates so that a resistance value of thefeedback resistor becomes a resistance value of the fixed resistor.Then, if the FET becomes ON with an increase in the light receivingpower, the preamplifier operates so that a combined resistance value ofthe fixed resistor and an ON resistor of the FET becomes a resistancevalue of the feedback resistor, i.e., a conversion gain of thepreamplifier.

It is noted that the preamplifier has a capacitor for phase compensationprovided in parallel with the feedback resistor, and by optimizing acapacity value of the capacitor, non-stability of the circuit due to anincrease in the light receiving power is suppressed.

However, the preamplifier disclosed in the above literature can ensurephase allowance indicating a level of stability on frequencycharacteristics of an open loop by the phase compensation, but, a gainpeak, in turn, occurs on frequency characteristics of a closed loop.This causes the conversion gain in a certain frequency band to increaseand an output voltage waveform to be distorted, called “pattern effect”.

In the preamplifier disclosed in the Non-patent literature 1, there is atime zone in which the conversion gain is large even though the lightreceiving power is high, during a transition state until the conversiongain becomes its stationary state. Therefore, an input terminal voltageof a load resistor becomes low in the negative-feedback amplifier in itstransition state, and an excessive current flows into an inputtransistor and the load resistor, which may cause element destruction.

The present invention has been achieved to solve at least theconventional problems, and it is an object of the present invention toprovide a preamplifier in which a waveform is less distorted and acontrol operation for a conversion gain is stable, without depending onwhether the operation state is the transition state or the stationarystate, and which has excellent wide dynamic range characteristics.

DISCLOSURE OF THE INVENTION

A preamplifier according to one aspect of the present invention includesa negative-feedback amplifier circuit that converts a current signaloutput from a light receiving element into a voltage signal; and aconversion-gain control circuit that simultaneously controls aresistance value of a feedback resistor portion of the negative-feedbackamplifier circuit and a resistance value of a load resistor portion ofthe negative-feedback amplifier circuit, based on the voltage signaloutput from the negative-feedback amplifier circuit. Each of thefeedback resistor portion and the load resistor portion includes a fixedresistor element, a metal-oxide-semiconductor field-effect-transistor(MOSFET) element, and a diode-connected transistor, connected inparallel.

According to the present invention, the negative-feedback amplifiercircuit that includes an inverting amplifier circuit and an outputbuffer converts the output current signal output from the lightreceiving element to the voltage signal, and the conversion-gain controlcircuit concurrently controls the resistance value of the feedbackresistor portion of the negative-feedback amplifier circuit and theresistance value of the load resistor portion thereof. Each of thefeedback resistor portion and the load resistor portion includes thefixed resistor element, the MOSFET element, and the diode-connectedtransistor, which are connected in parallel with one another. Resistancevalues of the feedback resistor portion and the load resistor portionare varied by conduction control of the MOSFET element or thetransistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a circuit configuration of a preamplifieraccording to a first embodiment of the present invention;

FIG. 2 is a diagram of input-output characteristics of the preamplifier;

FIG. 3 is a diagram of change characteristics of transimpedance withrespect to frequencies upon control over a feedback resistor;

FIG. 4 is a diagram of change characteristics of transimpedance withrespect to frequencies upon concurrent control for the feedback resistorand a load resistor;

FIG. 5 is a diagram of a circuit configuration of a preamplifieraccording to a second embodiment of the present invention;

FIG. 6 is a diagram of a circuit configuration of a preamplifieraccording to a third embodiment of the present invention;

FIG. 7 is a diagram of a circuit configuration of a preamplifieraccording to a fourth embodiment of the present invention; and

FIG. 8 is a diagram of a circuit configuration of a preamplifieraccording to a fifth embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Exemplary embodiments of a preamplifier according to the presentinvention are explained in detail below with reference to theaccompanying drawings. It is noted that the present invention is notlimited by the embodiments.

FIG. 1 is a diagram of a circuit configuration of a preamplifieraccording to a first embodiment of the present invention. A preamplifier1 of this figure includes an inverting amplifier circuit 2, an outputbuffer circuit 3, a feedback resistor portion 4, and a load resistorportion 5. The feedback resistor portion 4 includes a fixed resistorelement 41, a MOSFET 42, and a diode-connected transistor 43, which areconnected in parallel to one another. Likewise, the load resistorportion 5 includes a fixed resistor element 51, a MOSFET 52, and adiode-connected transistor 53, which are connected in parallel to oneanother. Further, connections are provided in these circuits as follows.That is, a base terminal of a transistor 62 that is an input terminal ofthe output buffer circuit 3 is connected to a collector terminal of atransistor 61 that is an output terminal of the inverting amplifiercircuit 2. An emitter terminal of the transistor 62 that is an outputterminal of the output buffer circuit 3 is connected to a base terminalof the transistor 61 that is an input terminal of the invertingamplifier circuit 2. These two are connected to each other in thefeedback resistor portion 4 to configure the negative-feedback amplifiercircuit. Furthermore, a bias voltage Vpd is applied to a cathodeterminal of a light receiving element 6, and an anode terminal thereofis connected to an input terminal of the preamplifier 1, i.e., the inputterminal of the inverting amplifier circuit 2. A predetermined controlvoltage is supplied to gate terminals of the MOSFET 42 and the MOSFET52. The control voltage is generated based on, for example, an output ofthe negative-feedback amplifier circuit.

Operation of the circuit of FIG. 1 is explained below. Details ofoperations of the MOSFET 42 and the diode-connected transistor 43 thatform the feedback resistor portion 4 and details of operations of theMOSFET 52 and the diode-connected transistor 53 that form the loadresistor portion 5 are explained later.

Referring to FIG. 1, an optical signal received in the optical receiver,which is not shown, is converted to a current signal in the lightreceiving element 6, and the current signal is converted to a voltagesignal in the preamplifier 1 that includes the inverting amplifiercircuit 2, the output buffer circuit 3, and the feedback resistorportion 4. In this type of preamplifier, transimpedance that is a ratioof an output voltage to an input current largely contributes toinput-output characteristics of the circuit.

In the preamplifier 1 having the circuit configuration, thetransimpedance is a resistance value of the feedback resistor portion 4,but the resistance value is controlled according to the light intensityof the optical signal received so that circuit saturation does notoccur. More specifically, if the light intensity of the optical signalreceived is low, then the resistance value of the feedback resistorportion 4 is controlled so as to increase, and if the light intensity ofthe optical signal received is high, then the resistance value of thefeedback resistor portion 4 is controlled so as to decrease.

In the preamplifier 1 according to the first embodiment, the resistancevalue of the load resistor portion 5 is also controlled concurrentlywith the feedback resistor portion 4. Note that the reason ofcontrolling the resistance value of the load resistor portion 5concurrently with the feedback resistor portion 4 is explained in detailbelow.

The input-output characteristics of the preamplifier 1 according to thefirst embodiment are explained below. FIG. 2 is a diagram of theinput-output characteristics of the preamplifier 1. More specifically,an output voltage of the preamplifier 1 with respect to the intensity oflight received of the light receiving element 6 is plotted by curvessuch as a characteristic curve when the resistance value of the feedbackresistor portion 4 is not controlled (K1: conversion gain notcontrolled) and another characteristic curve when the resistance valueof the feedback resistor portion 4 is controlled (K2: conversion-gaincontrolled).

As shown in the characteristic curve K1 of FIG. 2, if the resistancevalue of the feedback resistor portion 4 is not controlled, that is, ifthe resistance value of the feedback resistor portion 4 is a fixed valueand if the intensity of the light received is low, the output voltage islinearly amplified. However, if the intensity of the light receivedincreases, the output voltage enters a saturation region of the circuitas indicated by region A1, which causes waveform distortion to occur.Therefore, if the resistance value of the feedback resistor portion 4 isnot controlled, a sufficient dynamic range cannot be ensured.

On the other hand, in the preamplifier according to the firstembodiment, the resistance value of the feedback resistor portion 4 iscontrolled based on the intensity of the light received. Therefore, asshown in the characteristic curve K2 of FIG. 2, when the intensity ofthe light received is high, the conversion gain decreases, and theoutput voltage becomes low to be prevented from its entering thesaturation region of the circuit. Therefore, according to the firstembodiment configured to control the resistance value of the feedbackresistor portion 4, it is possible to ensure a sufficient dynamic rangewithout waveform distortion.

FIG. 3 is a diagram of change characteristics of transimpedance withrespect to frequencies upon control over the feedback resistor portion4. In this figure, a transimpedance value is represented by a decibelvalue. As shown by a characteristic curve K3 and a characteristic curveK4 of FIG. 3, if the resistance value of the feedback resistor portion 4decreases (high conversion gain (low conversion gain), a cutofffrequency fc of the preamplifier 1 increases, and a peak of gain occurson the frequency characteristics of a closed loop. As a result, thereoccurs a phenomenon such that the conversion gain in a frequency bandlower than the cutoff frequency increases which causes the outputvoltage waveform to be distorted. The distortion of the output voltagewaveform is well known as the pattern effect.

In the preamplifier 1 according to the first embodiment, the resistancevalue of the load resistor portion 5 is controlled concurrently with theresistance value of the feedback resistor portion 4 so that the cutofffrequency f_(c) becomes a fixed value. The characteristics at this timeare shown in FIG. 4. That is, FIG. 4 is a diagram of changecharacteristics of transimpedance with respect to frequencies uponconcurrent control over the feedback resistor portion 4 and the loadresistor portion 5. As shown by a characteristic curve K5 and acharacteristic curve K6 of FIG. 4, even if the resistance value of thefeedback resistor portion 4 changes, by controlling the load resistorportion 5, the cutoff frequency of the preamplifier 1 can be controlledto an almost fixed value. Therefore, it is possible to ensure phaseallowance and hold stability of the circuit upon conversion-gain controlwithout causing a gain peak to occur on the frequency characteristics ofthe closed loop. In other words, it is possible to obtain thepreamplifier excellent in the wide dynamic range characteristics withoutcausing waveform distortion to occur.

The contents above are explained below using equations. At first, acutoff frequency f_(c) of the preamplifier as a negative feedbackamplification type can be expressed by the following equation, whereR_(f) is a resistance value of the feedback resistor portion 4 betweenan input and an output of the inverting amplifier circuit, C_(i) is aninput capacity of the preamplifier, and G is an open-loop gain (gainwithout negative feedback) of the preamplifier 1.

$\begin{matrix}{f_{c} = \frac{G}{2{\pi \cdot R_{f} \cdot C_{i}}}} & (1)\end{matrix}$

As indicated in the above equation, in order to obtain a high conversiongain when the intensity of the light received is low, the resistancevalue R_(f) of the feedback resistor portion 4 is controlled toincrease. Conversely, in order to obtain a low conversion gain when theintensity of the light received is high, the resistance value R_(f)thereof is controlled to decrease. Therefore, as is apparent from theequation (1), the cutoff frequency f_(c) increases upon the lowconversion gain as compared with that upon the high conversion gain.

The open-loop gain G of the preamplifier 1 is a voltage gain of anemitter grounded amplifier that forms the inverting amplifier circuit 2,and is expressed by the following equation, where R_(e) is a resistancevalue of an emitter resistor 7 of the emitter grounded amplifier, andR_(c) is a resistance value of the load resistor portion 5.

$\begin{matrix}{G = \frac{R_{C}}{R_{e}}} & (2)\end{matrix}$

In the equation (1), the only thing not to change the cutoff frequencyf_(c) is to also concurrently change the open-loop gain G of thepreamplifier 1 as expressed in the equation (2), according to a changein the resistance value R_(f) of the feedback resistor portion 4. Inother words, by controlling the resistance value R_(c) of the loadresistor portion 5 according to the change in the resistance value R_(f)of the feedback resistor portion 4, the cutoff frequency f_(c) can becaused to remain within a predetermined range.

Referring back to FIG. 1, operations of the feedback resistor portion 4and the load resistor portion 5 are explained in detail below.

At first, it is considered the case where each of the feedback resistorportion 4 and the load resistor portion 5 includes only a MOSFET andtherefore no diode-connected transistor is provided in FIG. 1.

It is generally known that an equivalent resistance between the drainand source terminals of the MOSFET changes according to potential(potential at a gate terminal) applied to its gate terminal. Therefore,a resistance combined with the fixed resistor element 41 is made tochange by a control voltage that is applied to the gate terminal of theMOSFET 42 and is determined based on an output voltage changingaccording to the light intensity of the optical signal received, and aconversion gain is controlled thereby. Likewise, a resistance combinedwith the fixed resistor element 51 is made to change by a controlvoltage applied to the gate terminal of the MOSFET 52, and an open-loopgain is also controlled thereby. Therefore, the conversion-gain controlaccording to the intensity of the light received and stabilizationcontrol of the cutoff frequency can be concurrently performed.

However, a predetermined time is required until the control is settledonly when the conversion gain is controlled based on the control of agate terminal voltage of the MOSFET. And a state where the conversiongain is large is maintained during a transition state up to thestationary state. As a result, if the state is in the transition stateand the intensity of light received is high, circuit saturation mayoccur in the preamplifier 1. Furthermore, in some cases, the componentsforming the preamplifier 1 may be destroyed caused by overcurrent.

In addition to the MOSFET, a diode-connected transistor is thereforeadded to each of the feedback resistor portion 4 and the load resistorportion 5.

It is generally known that the diode-connected transistor enters aconducting state when a potential difference between the base terminaland the emitter terminal becomes about 0.8 volt or more, and that theequivalent resistance between the collector terminal and the emitterterminal decreases. Therefore, the diode-connected transistor functionsso that if the voltage across terminals applied to both ends of thefeedback resistor portion 4 becomes about 0.8 volt or more depending onthe light intensity of the optical signal received, the combinedresistance value of the feedback resistor portion 4 decreases and theconversion gain decreases. Likewise, it functions so that if the voltageacross terminals applied to both ends of the load resistor portion 5becomes about 0.8 volt or more, the combined resistance value of theload resistor portion 5 decreases and the open-loop gain decreases.

Therefore, for the conversion-gain control using the diode-connectedtransistors 43 and 53 as are the configurations of the feedback resistorportion 4 and the load resistor portion 5 of the first embodiment, if avoltage between the base terminal and the emitter terminal of each ofthe diode-connected transistors 43 and 53 becomes about 0.8 volt ormore, the diode-connected transistors 43 and 53 instantaneously operatebit by bit. This makes it possible to prevent circuit saturation anddestruction of the components of the preamplifier 1 even if the state isin the transition state as above and the intensity of the light receivedis high.

In the conversion-gain control only by the diode-connected transistor,the conversion gain decreases only when the intensity of the lightreceived is in a high level, and the conversion gain is maintained highwhen the intensity thereof is in a low level. As a result, an S/N ratio(signal-to-noise ratio) of the output voltage of the preamplifier 1 isdegraded, and there is concern that reception sensitivity may bedeteriorated.

In such a case as explained above, when the state of the conversion-gaincontrol is the stationary state, by controlling the gate terminalvoltage of the MOSFET so that a voltage across the terminals of thefeedback resistor portion 4 becomes about 0.8 or less, thediode-connected transistor is turned ON only when the state of theconversion-gain control is the transition state. Conversely, thediode-connected transistor is turned OFF in the stationary state, andtherefore, there is no concern that the reception sensitivity isdeteriorated.

As explained above, according to the preamplifier of the firstembodiment, it is configured to concurrently control the MOSFETs 42 and52 and the diode-connected transistors, based on the light intensity ofthe optical signal received, so that they are brought into conduction.Therefore, it is possible to obtain the preamplifier excellent in thewide dynamic range characteristics without occurrence of waveformdistortion while the circuit saturation and the destruction of thecomponents in the transition state are suppressed and the stability ofthe circuit upon conversion-gain control is maintained.

FIG. 5 is a diagram of a circuit configuration of a preamplifieraccording to a second embodiment of the present invention. In thisfigure, portions the same as or equivalent to these in FIG. 1 arerepresented by the same signs. As shown in FIG. 5, a reference voltageV_(ref0) is supplied to the base terminal of the diode-connectedtransistor 53 that forms the load resistor portion 5. It is assumed thata collector terminal voltage of the transistor 61 provided in theinverting amplifier circuit 2 is defined as an input terminal voltageV_(ref1). Based on this, the reference voltage V_(ref0) becomesequivalent to the collector terminal voltage upon no signal input, thatis, when the signal is not input to the inverting amplifier circuit 2.

Operation of the circuit as shown in FIG. 5 is explained below. If acurrent I₁ flows into the feedback resistor portion 4 when an opticalsignal with a predetermined light intensity is received, the inputterminal voltage V_(ref1) that is the collector terminal voltage of thetransistor 61 can be expressed by the following equation.V _(ref1) =V _(ref0) −R _(f) ×I ₁  (3)

A relation expressed in equation (3) can be considered as follows. Morespecifically, when the current I₁ flowing upon reception of the opticalsignal with the predetermined light intensity flows through the feedbackresistor portion 4, a potential at an emitter terminal of the transistor62 provided in the output buffer circuit 3 drops by R_(f)×I₁ from thepotential at the emitter terminal when no optical signal is input. Onthe other hand, the transistor 62 operates so that a predeterminedvoltage drop (about 0.8 volt) between the base terminal and the emitterterminal is maintained. Therefore, the collector terminal voltage of thetransistor 62 also drops by R_(f)×I₁. Since the voltage drop is causedby a change in the current flowing through the load resistor portion 5,a difference between the input terminal voltage Vref0 upon no signalinput and the input terminal voltage Vref1 when the current I1 flowsthrough the feedback resistor portion 4 becomes almost equal toR_(f)×I_(1.)

As shown in FIG. 5, therefore, the voltage between the base terminal andthe emitter terminal of the diode-connected transistor 43 that forms thefeedback resistor portion 4 is almost equal to the voltage between thebase terminal and the emitter terminal of the diode-connected transistor53 that forms the load resistor portion 5. In other words, when theoptical signal with the predetermined light intensity or higher isreceived, the circuit operates so that the diode-connected transistor 43that forms the feedback resistor portion 4 and the diode-connectedtransistor 53 that forms the load resistor portion 5 are made toconcurrently turn ON.

If the diode-connected transistor 43 that forms the feedback resistorportion 4 is in the ON state and the diode-connected transistor 53 thatforms the load resistor portion 5 is in the OFF state, the circuitoperates in a state where the open-loop gain is high, although theconversion gain is low. This causes the phase allowance not to beensured and the circuit to oscillate, that is, the operation of thecircuit may become unstable.

Conversely, if the diode-connected transistor 43 that forms the feedbackresistor portion 4 is in the OFF state and the diode-connectedtransistor 53 that forms the load resistor portion 5 is in the ON state,the open-loop gain is low although the conversion gain is high.Therefore, a cutoff frequency becomes low, which may cause an adequateoutput voltage waveform not to be obtained.

According to the second embodiment, however, if the light intensity ofthe optical signal received is high, the circuit operates so that thediode-connected transistor 43 that forms the feedback resistor portion 4and the diode-connected transistor 53 that forms the load resistorportion 5 are made to almost concurrently turn ON. Thus, it is possibleto obtain the preamplifier excellent in the wide dynamic rangecharacteristics by suppressing waveform distortion without loss of thestability of the circuit.

FIG. 6 is a diagram of a circuit configuration of a preamplifieraccording to a third embodiment of the present invention. In thisfigure, portions the same as or equivalent to these in FIG. 1 arerepresented by the same signs. As shown in FIG. 6, areference-bias-voltage generating circuit 8 includes a circuitequivalent to the preamplifier 1 of FIG. 1, i.e., the negative-feedbackamplifier circuit. Further, a collector terminal of a transistor 81 thatis an output terminal of the inverting amplifier circuit that forms thereference-bias-voltage generating circuit 8 is connected to the baseterminal of the diode-connected transistor 53.

Operation of the circuit as shown in FIG. 6 is explained below. Thereference-bias-voltage generating circuit 8 includes circuits equivalentto the basic components of the preamplifier 1 excluding thereference-bias-voltage generating circuit 8. Therefore, a voltage at thecollector terminal of the transistor 81 that is an output terminal ofthe reference-bias-voltage generating circuit 8 becomes almostequivalent to the input terminal voltage (the voltage at the collectorterminal of the transistor 61 that is the output terminal of theinverting amplifier circuit 2) V_(ref0) upon no signal input.

It is noted that values of V_(ref0) fluctuate depending on fluctuationsin a supply voltage and fluctuations in ambient temperature. However,the reference-bias-voltage generating circuit 8 includes the circuitsequivalent to the basic components of the preamplifier 1 excluding thereference-bias-voltage generating circuit 8. Therefore, it is controlledso that the change in V_(ref0) due to the fluctuation in the supplyvoltage and the fluctuation in the ambient temperature is cancelled andthe voltage between the base terminal and the emitter terminal of thetransistor 53 is made to be a fixed value.

As explained above, according to the third embodiment, if the lightintensity of the optical signal received is high, regardless offluctuations in the supply voltage and fluctuations in the ambienttemperature, the circuit operates so that the diode-connected transistor43 that forms the feedback resistor portion 4 and the diode-connectedtransistor 53 that forms the load resistor portion 5 are made toconcurrently turn ON. Thus, it is possible to obtain the preamplifierexcellent in the wide dynamic range characteristics by suppressingwaveform distortion without loss of the stability of the circuit.

FIG. 7 is a diagram of a circuit configuration of a preamplifieraccording to a fourth embodiment of the present invention. In thisfigure, portions the same as or equivalent to these in FIG. 6 arerepresented by the same signs. As shown in FIG. 7, the output terminalof the reference-bias-voltage generating circuit 8 and the base terminalof the diode-connected transistor 53 are connected to each other througha voltage follower circuit 9.

Operation of the circuit as shown in FIG. 7 is explained below. An idealinput impedance of the voltage follower circuit 9 including anoperational amplifier is infinitely large, and an ideal output impedancethereof is infinitely small. More specifically, by connecting thereference-bias-voltage generating circuit 8 to the base terminal of thediode-connected transistor 53 through the voltage follower circuit 9,the circuit can be made to operate in an ideal manner without change inoperation conditions of the reference-bias-voltage generating circuit 8and the negative-feedback amplifier circuit.

As explained above, according to the fourth embodiment, if the lightintensity of the optical signal received is high, regardless offluctuations in the supply voltage and fluctuations in the ambienttemperature, the circuit operates so that the diode-connected transistor43 that forms the feedback resistor portion 4 and the diode-connectedtransistor 53 that forms the load resistor portion 5 are made toconcurrently turn ON under the ideal conditions. Thus, it is possible toobtain the preamplifier excellent in the wide dynamic rangecharacteristics by suppressing waveform distortion without loss of thestability of the circuit.

FIG. 8 is a diagram of a circuit configuration of a preamplifieraccording to a fifth embodiment of the present invention. In thisfigure, portions the same as or equivalent to these in FIG. 7 arerepresented by the same signs. The preamplifier 1 of FIG. 8 includes amean-value detecting circuit 10 that generates a mean value of outputvoltage signals of the output buffer circuit 3, an operational circuit11 a that generates a gate terminal voltage for controlling a resistancevalue of the MOSFET 42 that forms the feedback resistor portion 4 basedon the output of the mean-value detecting circuit 10, and an operationalcircuit 11 b that generates a gate terminal voltage for controlling aresistance value of the MOSFET 52 that forms the load resistor portion 5based on the output thereof, and these operational circuits 11 a and 11b output the respective gate terminal voltages.

Operation of the circuit as shown in FIG. 8 is explained below. Themean-value detecting circuit 10 detects and outputs a mean value ofoutput voltage signals of the output buffer circuit 3 that changeaccording to the intensity of light received. The operational circuit 11a generates and outputs the gate terminal voltage to be applied to theMOSFET 42 in order to decide a combined resistance value of the feedbackresistor portion 4 based on the output of the mean-value detectingcircuit 10. Likewise, the operational circuit 11 b generates and outputsthe gate terminal voltage to be applied to the MOSFET 52 in order todecide a combined resistance value of the load resistor portion 5 basedon the output of the mean-value detecting circuit 10.

As shown in FIG. 8, a feedback loop is formed with the negative-feedbackamplifier circuit including the inverting amplifier circuit 2 and theoutput buffer circuit 3, the mean-value detecting circuit 10, and theoperational circuits 11 a and 11 b. This formation causes the output toflexibly follow the changes in the intensity of the light received.

As a circuit (sensor) that detects an output of the output buffercircuit 3, for example, a case as follows is studied. The case is suchthat a feedback loop is formed with a low-peak detecting circuit but notwith the mean-value detecting circuit as shown in FIG. 8. An ordinarylow-peak detecting circuit has a characteristic such that it holds a lowpeak value when the intensity of the light received changes in adirection in which the intensity becomes lower, although it has anexcellent following capability. Therefore, the low-peak detectingcircuit has a defect such that the following capability is deterioratedwhen the intensity of the light received is low. Furthermore, thelow-peak detecting circuit has also a defect such that a mean valuedeviates from an actual mean value if an extinction ratio is low whenthe mean value is to be detected from medium values in an OFF level ofthe optical output, although no problem occurs if the extinction ratioof the optical signal is infinite.

However, as shown in the fifth embodiment, if the mean-value detectingcircuit is used as the circuit (sensor) that detects an output of theoutput buffer circuit 3, the output can be made to flexibly followchanges in light intensity of the optical signal received, and anaccurate mean value can be always output, which makes it possible toresolve the defects.

As explained above, according to the fifth embodiment, the output isallowed to accurately and flexibly follow the changes in the lightintensity of the optical signal received. Thus, it is possible to obtainthe preamplifier excellent in the wide dynamic range characteristics bysuppressing waveform distortion without loss of the stability of thecircuit.

INDUSTRIAL APPLICABILITY

The preamplifier according to the present invention is useful as apreamplifier for the optical communication systems and opticalreceivers, and particularly, the preamplifier is suitable for the casewhere the wide dynamic range characteristics are desired to be ensuredin the optical communication systems and the optical receivers.

1. A preamplifier comprising: a negative-feedback amplifier circuit thatconverts a current signal output from a light receiving element into avoltage signal; and a conversion-gain control circuit thatsimultaneously controls a first resistance value of a feedback resistorportion of the negative-feedback amplifier circuit and a secondresistance value of a load resistor portion of the negative-feedbackamplifier circuit, based on the voltage signal output from thenegative-feedback amplifier circuit, wherein each of the feedbackresistor portion and the load resistor portion includes a fixed resistorelement, a metal-oxide-semiconductor field-effect-transistor element,and a diode-connected transistor, connected in parallel.
 2. Thepreamplifier according to claim 1, wherein a base terminal voltage of atransistor that forms the load resistor portion is substantially samevoltage as an output voltage of an inverting amplifier circuit thatforms the negative-feedback amplifier circuit when the current signal isnot output.
 3. The preamplifier according to claim 2, further comprisinga reference-bias-voltage generating circuit that is formed with acircuit equivalent to the negative-feedback amplifier circuit, whereinan output terminal of the reference-bias-voltage generating circuit isconnected to a base terminal of the transistor that forms the loadresistor portion.
 4. The preamplifier according to claim 3, wherein avoltage follower circuit is connected between the output terminal of thereference-bias-voltage generating circuit and the base terminal of thetransistor that forms the load resistor portion.
 5. The preamplifieraccording to claim 1, wherein the conversion-gain control circuitincludes a mean-value generating circuit that generates a mean value ofthe voltage signal output from the negative-feedback amplifier circuit;and an operational circuit that converts an output voltage of themean-value generating circuit into a control voltage for controllingeach of the first resistance value and the second resistance value.